A front fork for a motorcycle or a bicycle may be subjected to wheel speeds in the whole range of 0-10 m/s and stroke lengths of up to 300 mm. In order to be able to absorb such high speeds and such large strokes, great demands are made of the front fork. It must be able to absorb forces and be strong, while at the same time it must be able to handle a large flow of oil. It is also desirable to have good control in the whole range of speeds and for the damping to be adjustable. A compact and light system that may be adapted to fit several different front fork dimensions is also required. Reference is made, for example, to patent U.S. Pat. No. 6,260,832, that shows a front fork of the type described above. U.S. Pat. No. 6,260,832 does not, however, have the desirable build-up of pressure that is described below.
A common type of damping systems may be represented by a damper of the De Carbon type, see for example FR1055443A, and have a serial damping force construction that is based on a principle of pressurizing two locations in series in order to avoid cavitation or the admixture of air into the damping medium. This system has limitations in that the pressures in the two pressurizing locations must more or less harmonize with each other, as the drop in pressure ([Delta]Pl=Piow-Pmid, [Delta]P2=Pmidt-Pgas) across the two pressurizing pistons should be greater than zero in order not to create cavitation. As the drop in pressure across the piston is dependent upon the flow resistance through the piston in combination with the force that acts on the piston, the flow resistance, controlled for example by a shims stack, may only be adjusted within a certain limited range which thus also results in a limited area of use for the damper. It is then also necessary to dimension pistons, piston rod and damping tubes so that the force absorption agrees with the pressures that have been built up, in order to obtain the required damping. With serial damping, the oil is forced through both the valves in series, which results in high flow speeds. With high flow speeds and high piston speeds, the design of the pistons is limited in order not to obtain an unwanted uncontrollable build-up of pressure due solely to the flow resistance.
A system with parallel damping solves the abovementioned problem. Examples of such dampers can be found in the patent documents EP1505315A2 and EP0322608A2. The parallelism in the damping arises through the damping medium being pressurized by two pressurizing pistons that are arranged parallel to each other in the damping chamber and in a space arranged outside the damping chamber. The pressurized outer space is interconnected with both the compression chamber and the return chamber. With parallel damping, the pressure on the low-pressure side of the damping piston is always as large as possible, irrespective of whether the front fork is subjected to a compression or a return stroke. The definition of the low-pressure side of the damping piston is the side of the piston where the volume of the chamber increases. Due to the fact that the pressure is never allowed to become zero on that side, cavitation is prevented. This parallel arrangement also means that the damper can be pressurized and the pressure, that is the damping, may be adjusted, without having to take into account the drop in pressure across the piston(s). The increase in pressure, as well as the increase in force, now takes place without cavitation, irrespective of the setting.
The designs according to EP1505315A2 and EP0322608A2 are adapted for shock absorbers that are not subjected to the same forces and impacts as a front fork.
EP1937995B1, filed by the applicant, solves this problem and discloses a device that comprises adjustable parallel damping, adapted to the demands of a front fork. The damping system disclosed comprises two concentric tubes in the form of a damping tube and an outer tube that is arranged around the damping tube that together form a removable insert system in the front fork. The insert system creates a double tube function in which the damping medium can flow in parallel as a result of the duct between the damping tube and the outer tube being used to connect together the two chambers and the common pressurizing location. This insert system forms a compact unit that is simple to adapt to different front fork dimensions and that can also be used as a kit for providing an existing front fork with parallel damping.
However, in recent years, advanced damping systems have come to be employed to a large extent also in the field of bicycles, more specifically the field of terrain bicycles, or mountain bikes. These applications place high demands on aspects of damping systems previously not quite as important, such as versatility, possibilities for aftermarket mounting, and most important place very high demands on reducing the weight of all components as much as possible. A device is thus required for a front fork of a bicycle that comprises adjustable parallel damping while at the same time minimizing the weight added to the bicycle. It is also advantageous if the device is able to be adjusted to suit different front fork dimensions and is adapted to be used as a kit for modifying an existing front fork.